Polytrimethylene ether glycol, also known as polyoxytrimethylene glycol or poly(1,3-propylene glycol), can be produced from 1,3-propanediol. Polytrimethylene ether glycols have primary hydroxyl groups, low melting points and are highly flexible. These polymers are useful in many applications, such as the production of elastomeric polymers and textile fibers. Polytrimethylene ether glycols are typically produced by the acid-catalyzed polycondensation of 1,3-propanediol, optionally in the presence of comonomer diols, as described in U.S. Pat. Nos. 6,977,291, 3,326,985 and 2,520,733.
It is known that polytrimethylene ether glycols produced by this acid-catalyzed polycondensation reaction can have quality problems, in particular a high percentage of unsaturated end groups and high color.
Since polytrimethylene ether glycols suitable for use as soft segment in elastomeric polymers such as polyurethanes must be capable of reacting with other monomers such as diisocyanates, it is required that they possess a high dihydroxy terminal functionality and especially a low percentage of unsaturated end groups. In addition, polytrimethylene ether glycols or copolymers thereof with high unsaturation are not acceptable for many end-uses, such as textile fiber applications as the unsaturated molecules either cause an undesired termination in polymerization reactions that use polytrimethylene ether glycol as a reaction component or the unsaturated molecules are not incorporated into the polymer at all, both leading to undesired polymer properties. The degree of unsaturation of the polymer depends primarily on reaction temperature, catalyst type, catalyst concentration and reaction time.
As described in U.S. Pat. No. 3,326,985, attempts to prepare higher molecular weight polytrimethylene ether glycols and copolymers thereof having a low content of unsaturated end groups have either been unsuccessful or required very long reaction times under mild reaction conditions. It was found that to produce polytrimethylene ether glycols having an average molecular weight of from 1000 to 1500 g/mole and a low content of unsaturated end groups, undesirably long reaction times were required. Attempts to shorten the reaction time by employing higher temperatures and/or larger concentrations of catalyst resulted in polymers of high unsaturation.
U.S. Pat. No. 3,326,985 disloses a method for producing polytrimethylene ether glycols of a molecular weight between 1200 and 1400 g/mole with unsaturation of about 20 meq/kg. This was achieved by using polytrimethylene ether glycols of a lower molecular weight and employing a process comprising vacuum stripping at high temperature (about 220-240° C.) and low pressure (about 0.1 to 1.1 kPa). This process contains several limitations, e.g., (1) the process yields a product with high unsaturation (about 20 meq/kg), (2) the molecular weight achieved is limited to a narrow range of 1200-1400 g/mole, (3) the process results in a significant yield loss caused by stripping off a volatile fraction and (4) the process requires costly vacuum-rated equipment and a vacuum pump to achieve the low pressure required.
Acid catalyzed polymerizations generally depend upon a complex number of factors, such as the nature of the reaction, catalyst type, catalyst concentration, solvent effects such as water content and reaction temperature. Higher temperatures generally favor shorter reaction times which is preferable from a practical point of view. However, higher temperatures also favor side product formation, especially formation of unsaturated end groups and color. The present invention discloses the unexpected finding that a reaction product having low unsaturation, and, in some embodiments, lower color, can be produced at a high reaction rate.